There is an increasing need for optical fiber communications systems having higher data rates. One way in which laser radiation can be modulated at a high speed, for example in excess of 1 Gbits/s, is with the use of a distributed feedback (DFB) electro-absorption modulator (EAM). Such modulators may be integrated with a laser source on a semiconductor chip.
The bandwidth of a DFB-EAM integrated laser modulator (ILM) may be limited by parasitic capacitance in the EAM. A further problem arises in that such parasitic capacitance may be variable between devices produced from the same production process. Nominally identical devices may therefore have a range of maximum operating bandwidths, and hence maximum achievable data rates. For DFB-EAM devices operating at 10 Gbits/s a target system optical rise time (20% to 80%) would be about 39 ps, corresponding to a nominal parasitic capacitance of about 0.7 pF (with a 25 Ohm load and an 8.8 GHz bandwidth driver). A typical manufacturing variation on the nominal capacitance would be ±20%, giving a corresponding ±10% variation in the rise time.
An ideal modulator has a sharp transition between on and off states. In practice the EAM device has a finite bandwidth so the transitions are rounded and the rise time is increased. One solution to this problem is to apply comparatively more modulation power to the higher frequency components in the modulation signal, thus compensating for the roll off in the frequency response and speeding-up the transitions. However, in an EAM device, rapid changes in the output power cause a further problem, known as “chirp”, which is a shift in wavelength due to a refractive index change in the semiconductor material of the modulator that accompanies changes in its absorption. Changes in wavelength result in increased dispersion in an optical fiber transmission line, which closes the so-called “eye pattern” of the modulated signal and increases the received bit error rate. Chirp therefore causes a loss of some or all of the potential benefit that might have been had from driving the electro absorption modulator at a higher power for those higher frequency components associated with sharp transitions in the modulated signal.
Conventional approaches to these problems involve designing the DFB-EAM device to minimize parasitic capacitance and to minimize chirp. Because of manufacturing variability, the modulation driver circuits are then designed to maximize the data rate for an average manufactured device. All of these approaches have their limitations.
It is an object of the present invention to address these issues.